The present invention relates to an optical fiber amplifier for clamping and equalizing gain in optical communication systems, and particularly to an optical fiber amplifier for clamping and equalizing gain, capable of clamping gain automatically by maintaining the same population inversion, and capable of maintaining the same gain equalization for the wide signal gain band to achieve the high transmission capacity, in the case that the number of channel varies in the input signal.
Generally, in optical communication systems, an optical amplifier is used to compensate loss in a signal. Such an optical amplifier is a device for compensating loss of attenuation of the optical signal due to a long distance transmission.
Optical fiber amplifiers are widely used in optical communication systems. Especially, an erbium doped fiber amplifier (EDFA) is used to amplify a signal periodically in certain distance so as to compensate the attenuation of optical signal caused by the long distance transmission, when a great number of data are transmitted over long distance, without being regenerated through a strand of optical fiber.
If the wavelength division multiplexing (WDM) optical transmission system is used, the whole transmission capacity increases rapidly in proportion to the number of wavelengths used, since a number of data can be transmitted on many carriers of different wavelength each other.
In the WDM network, changes in the number of channels frequently occur on account of changes in network configuration, malfunctions in constituent elements, and frequent add/drops of channels. Change of the total power in the input signal associated with the above change can induce a gain change and a transient to the output of the remaining optical channels. As a result, transmission errors can increase due to the above mentioned gain variations. Accordingly, in WDM networks, the gain variation on account of a change of the number of input signal must be minimized.
There are two typical methods in automatic gain clamping for an optical fiber amplifier. One method is by adjusting the pumping light power and the other is by applying a compensating signal. Recent studies on all-optical automatic gain clamping methods show the relatively low output power and the equalized gain band of approximately 16nm.
However, recent trends are to employ more than 32 channels and the total output power should be considerably high in proportion to the number of channels, since the signal power at each channel is almost the same. In this case, the sufficient output power cannot be produced with existing configurations of gain-clamped optical fiber amplifier. In addition, since the equalized gain band must be sufficiently wide and the equalizing degree must be quite good for long distance transmissions, a gain-equalizing filter is inevitably required. Since the amplifier can be used in WDM networks, only after solving such problems simultaneously, the development of such an amplifier is very important. The present invention provides an optical fiber amplifier with a simple and reliable configuration capable of solving such problems.
FIG. 1 is a block diagram showing an optical fiber amplifier for clamping and equalizing gain of a prior art.
As shown in FIG. 1, the amplifier comprises: a first isolator 21 for passing the input signal light only in one direction; a first optical coupler 11 for coupling the outputs of the first isolator 21 and a filter 31; a pump 71 for producing the population inversion in an amplifying medium (EDF) 41 by increasing the pump power; a second optical coupler 12 for coupling the outputs the first optical coupler and the pump 71; an amplifying medium 41 for amplifying the input signal from the second optical coupler 12; a second isolator 22 for blocking the reflected light and passing the transmitted light in the output from the amplifying medium 41; a third optical coupler 13 for allotting the output of the second isolator 22; a third isolator 23 for isolating the allotted signals at the third optical coupler 13; a filter 31 for passing only a specified wavelength among the output from the third isolator 23 and transmitting it to the first optical coupler 11; and a fourth isolator 24 for isolating the allotted signals at the third optical coupler 13 after inputting it, and outputting the amplified optical signal, in conjunction with a compensating signal.
As mentioned above, the construction of the optical fiber amplifier of FIG. 1 is for applying a compensating signal.
That is, an amplifying medium for amplifying the input signal from a second optical coupler 12, a pump to provide the population inversion in the amplifying medium 41, two optical couplers 11, 13 of the ring shaped construction for applying a compensating signal, and a filter 31 to transmit only a specified wavelength are included in the optical fiber amplifier for clamping gain of the prior art.
The gain of the optical fiber amplifier for clamping and equalizing gain of the prior art is determined by the losses of two ring shaped couplers 11, 13. When the power of the pump 71 is increased to increase the output signal power, only the laser signal used as a compensating signal is amplified without changing the power of the amplified signal output. Also, the output power will be decreased due to the loss of the third optical coupler 13.
Eventually, the optical fiber amplifier for clamping gain of the prior art shown in FIG. 1 cannot be applied to the WDM system because the equalization of gain is not considered.
FIG. 2 is a block diagram showing an optical fiber amplifier of another example of a prior art that adds a gain equalizing filter to the output part of the optical fiber amplifier in FIG. 1.
As shown in FIG. 2, the amplifier comprises: a first isolator 21 for passing the input signal light only in one direction; a first optical coupler 11 for coupling the outputs of the first isolator 21 and a filter 31; a pump 71 for producing the population inversion in an amplifying medium (EDF) 41 by increasing the pump power; a second optical coupler 12 for coupling the outputs the first optical coupler and the pump 71; an amplifying medium 41 for amplifying the input signal from the second optical coupler 12; a second isolator 22 for blocking the reflected light and passing the transmitted light in the output from the amplifying medium 41; a third optical coupler 13 for allotting the output of the second isolator 22; a third isolator 23 for isolating the allotted signals at the third optical coupler 13; a filter 31 for passing only a specified wavelength among the output from the third isolator 23 and transmitting it to the first optical coupler 11; a gain equalizing filter 51 for equalizing the gain of the output of the allotted signal from the third optical coupler 13; and a fourth isolator 24 for isolating the output of the gain equalizing isolator 51, and outputting the optical signal with an additional compensating signal.
The optical fiber amplifier including a gain equalizing filter of a prior art as shown in FIG. 2 simply equalizes and clamps the gain by providing the gain equalizing filter at the output terminals of the optical fiber amplifier.
According to the construction of FIG. 2, an equalized gain band of approximately 30 nm can be obtained from 1530-1560 nm.
However, in the case of the optical fiber amplifier including a gain-equalizing filter, there is a problem of the relatively large loss for the output of the signal since the filter is located at the output terminals.
FIG. 3 is a block diagram showing an optical fiber amplifier of still another example of a prior art.
As shown if FIG. 3, the amplifier comprises: a first isolator 21 for passing the input signal light and isolating the light coming from the opposite direction; a first pump 71 for producing the population inversion in an amplifying medium 41 by increasing the pump power; a second optical coupler 12 for coupling the outputs of the first isolator 21 and the first pump 71; a first amplifying medium 41 for amplifying the input signal from the second optical coupler 12; a second isolator 22 for isolating the output from the first amplifying medium 41; a gain equalizing filter 51 for equalizing the gain of the input signal from the second isolator 22; a second pump 72 for producing the population inversion in a second amplifying medium 42 by increasing the pump power; a third optical coupler 13 for coupling the outputs of the gain equalizing filter 51 and the second pump 72; a second amplifying medium 42 for amplifying the input signal from the third optical coupler 13; and a fourth isolator 24 for isolating the output of the second amplifying medium 42 and outputting the signal of the equalized gain.
Thus, in such an optical fiber amplifier for equalizing gain, by providing a gain-equalizing filter 51 between the first and the second amplifying stages, an equalized gain of more than 30 nm and high output can be obtained.
FIG. 4 is a block diagram showing an optical fiber amplifier for clamping gain, combined with the optical fiber amplifier for equalizing gain of FIG. 3.
As shown in FIG. 4, the amplifier comprises: a first isolator 21 for passing the input signal light and isolating the light coming from the opposite direction; a first optical coupler 11 for coupling the outputs of the first isolator 21 and a filter 31; a first pump 71 for producing the population inversion in an amplifying medium 41 by increasing the pump power; a second optical coupler 12 for coupling the outputs of the first coupler 11 and the first pump 71; a first amplifying medium 41 for amplifying the input signal from the second optical coupler 12; a second isolator 22 for isolating the output from the first amplifying medium 41; a gain equalizing filter 51 for equalizing the gain of the input signal from the second isolator 22; a second pump 72 for producing the population inversion in a second amplifying medium 42 by increasing the pump power; a third optical coupler 13 for coupling the outputs of the gain equalizing filter 51 and the second pump 72; a second amplifying medium 42 for amplifying the input signal from the third optical coupler 13; and a fourth optical coupler 14 for allotting the optical output signals from the second amplifying medium 42; a third isolator 23 for isolating the output of the fourth optical coupler 14; a filter 31 for passing light of only a specified wavelength from the third isolator 23 and transmitting it to the first optical coupler 11; and a fourth isolator 24 for isolating the output of the fourth optical coupler 14 and outputting the signal of the clamped and equalized gain.
The optical fiber amplifier for clamping gain of FIG. 4, in combination of the idea in an optical fiber amplifier for equalizing gain can obtain the equalized and clamped gain with relatively high output power, since the two stages of construction is provided with a gain equalizing filter 51 between two stages.
However, there is the same problem as in the case of FIG. 1. That is, a high output power is difficult to obtain as the fourth optical coupler 14 is provided at the output terminal and acts as a loss provider.
As described above, in the optical fiber amplifier of prior arts or the optical fiber amplifier combined with two technologies above mentioned, it is difficult in obtaining a sufficient gain-clamped and gain-flattened output power since the obtainable clamped gain is limited. Also, as a filter is provided after the last stage of the amplifier to equalize gain, there are a problem of a decreased gain and an increased output power.
In an optical fiber amplifier for clamping gain, no results have been reported to obtain a gain band of more than 30nm. And even in the modified examples that employed the extension of the ideas, there is still a difficulty in obtaining good characteristics.
At present, the number of wavelength channels is growing to increase the transmission capacity, and a wide gain band of more than 30 nm has already been used in the long distance transmission systems. However, so far the optical fiber amplifier for WDM network systems has not been developed to satisfy the high output power with gain clamping and gain flattening, in the case of variations in the number of channels.
Accordingly, the present invention is made to solve the problems above mentioned, and an object of the present invention is to provide an optical fiber amplifier for clamping and equalizing gain in the optical communication system, capable of clamping gain automatically by maintaining the same population inversion, and capable of maintaining the same gain equalization for the wide signal gain band to achieve the high transmission capacity, in the cases that the number of channel varies in the input signal.
Another object of the present invention is to provide an optical fiber amplifier for clamping and equalizing gain, which can supply a high output power required for many WDM systems, while clamping gain for each channel and providing the equalized gain band width of more than 30nm.
Next, the construction of the optical fiber amplifier for clamping and equalizing gain in optical communication systems of the present invention will be described with the attached drawings.
FIG. 5 is a block diagram showing the optical fiber amplifier for clamping and equalizing gain in optical communication systems of the present invention.
As shown in FIG. 5, the optical fiber amplifier for clamping and equalizing gain in optical communication systems comprises: a gain clamping amplifier unit 100 for clamping gain, so as to make input signals have the same output for each channel by inputting optical signal and applying a compensating signal; a second isolator 22 for passing the signal light input from said gain clamping amplifier unit 100 and blocking the light coming from the opposite direction; a gain equalizing filter 51 for equalizing the output gain spectrum from the input signal from said second isolator 22; and a post-amplifier unit 200 for amplifying the signal from said gain equalizing filter 51.
Said gain clamping amplifier unit 100 inputs a compensating signal by a ring laser constructed by connecting both ends of an amplifying medium.
Alternatively, said gain clamping amplifier unit 100 applies a compensating signal by constructing a laser resonator using a FBG (Fiber Bragg Grating) pair.
Otherwise, said gain clamping amplifier unit 100 applies a compensating signal by using an additional LD (laser diode).
Said gain equalizing filter 51 is constructed by considering the gain characteristics of said gain clamping amplifier unit 100 and post-amplifier unit 200.
Said post-amplifier unit 200 is made up of a pumping configuration in the forward direction so that a high output power can be obtained.
Alternatively, said post-amplifier unit 200 is made up of a pumping configuration in the backward direction so that a high output power can be obtained.
Otherwise, said post-amplifier unit 200 is made up of a pumping configuration in both directions so that a high output power can be obtained.
FIG. 6 is a detailed diagram of FIG. 5.
As shown in FIG. 6, said gain clamping amplifier unit 100 includes: a first pump 71 for producing the population inversion in an amplifying medium 41 by increasing the pump power; a first optical coupler 11 for coupling said input signal and the output of said first pump 71; a first amplifying medium (EDF1) 41 for amplifying the input signal from said first optical coupler 11; and a laser resonator system for producing a compensating signal and thus clamping the gain of the first stage.
Said gain clamping amplifier unit 100 further includes a first isolator 21 for passing the input signal light and blocking the light coming from the opposite direction.
Said post-amplifier unit 200 includes: a second pump 72 and a third pump 73 for producing the population inversion in a second amplifying medium 42 by increasing the pump power; a second optical coupler 12 for coupling the outputs from said gain equalizing filter 51 and said second pump 72; a second amplifying medium (EDF2) 42 for amplifying the input signal from said second optical coupler 12; and a third optical coupler 13 for coupling the outputs from said second amplifying medium 42 and said third pump 73.
Said post-amplifier unit 200 further includes a third isolator 23 for isolating the output from said third optical coupler 13 and outputting the gain clamped and equalized signal.